Combining circuit



Feb. 7, I195@ H. B. SMITH fi fi COMBINING CIRCUIT Filed Dec. 24, 1946 Pilaf mama .H/S- ATTOEMX' Patented Feb. 7, i950 COMBINING CIRCUIT Harry B. Smith, Hyattsville, Md. Application December 24, 1946, Serial No. 718,249

(Granted under the act of March 3, 1883, as amended April 30, 1928; 370 0. G.-757) 2 Claims.

, This invention is in electrical apparatus and more particularly is a novel mixing or combining circuit.

The primary object of the invention is to provide a circuit for combining two Baudot electrical signals to obtain a third Baudot type signal, the combining being in accordance with the so-called Vernam rule (see the patent to Vernam, No. 1,310,719). According to this rule, the following results may be obtained by adding two Baudot signalsv representing a marking signal and a spacing signal).

Another object is to provide a circuit for combining two electrical signals and indicating a similarity or difference in their amplitudes.

Other objects will be apparent from a reading of the following specification and claims.

In the drawings:

Figure 1 is a block diagram illustrating the basic features of the invention; and

Figure 2 is a schematic diagram of the mixer stage of Figure 1.

It will be understood that the invention is primarily concerned with mark-space or on-ofi signals, and it is to such signals that reference will be made in the following description, although signals of any two desired levels may be combined.

With reference to the drawings and especially to Figure 1, control stages In and II are identical and may herein be considered to comprise any means for supplying on-off or two-level signals of the Baudot type. If speeds of the order of those encountered in telegraph work are to be handled, the control stages may conveniently operate with standard perforated tapes and relays. At higher speeds, tubes will be employed. The signals are combined in mixer stage |2.

According to Figure 2, the mixer stage includes two tubes l5 and I6 (which may be enclosed in a single envelope), these serving as switching tubes for the mixer proper. High-mu triodes are shown, but high-gain pentodes may be employed. Tube i5 is biased in such fashion that one signal from its control stage ill (a marking signal, for example) is more positive than its grid bias, whereas the other signal (spacing) is less positive than said grid bias. Tube I6 is similarly biased with respect to the signals from its control stage The output of tube I5 is taken from the plate (tube l8) and a signal, similarly taken from tube i6 and fed to the grid of tube IS (the other half of the mixer), is used to control the latter-mentioned tube.

The two switching stages I5 and I6 are identi-- cal so that plate resistors 20 and 2| are of like value as are resistors 22 and 23. The result is that under conditions of no signal the voltage at the plate of tube l5 equals that at the plate of tube l6 while the voltage at point 24 equals that at point 25. At no signal, therefore, the cathodes of tubes l8 and H) are equally more positive thantheir grids by amounts represented by the drops across resistors 20 and 2|. The value of resistor 22 is so proportioned with respect to the value of 2B, and that of 23 with that of 2|, that regardless of the degree of conductivity of switching tubes l5 and IS, the voltage drops across plate resistors 20 and 2| are sufficient to hold mixing tubes l8 and i9 cut ofi.

Similarly, if spacing signals are applied simultaneously to tub s l5 and I6. the voltage drops across re istors 20 and 2| wil be equal and, while smaller than before. still not small enough to raise the grids of tubes l8 and I9 above c t oil.

In either case. the output of the mixing circuit remains at the same figure, fixed by the values of the various circuit components.

If, ho e er, a m rking si nal is applied to the r d 0" tube 5 whil a s acing signal is appl ed to the gr'd of tube "5. th voltage drop across resistor 2D is larger than at no signal while the drop across resistor 2| is smaller than at no signal. Consequently. the voltage at the plate of tube I5 is small r, that is to say. less positve than the vo tage at the plat" of tube !6, and tube IS (with ts cathode less positive and its grid more pos tive th n normally) wil conduct to saturation. Conversely, if switching tube l6 conducts while a spacing signal is appied to tube Hi, mixing tube l8 conducts while tube l9 remains cut off. In either case, the output of the mixing circuit assumes a new value, lower than before.

The circuit illustrated can readily handle not only mark-space signals but signals of randomly varying amplitudes as well. In the latter case, it will be desirable to calibrate the output of the mixer stage. If necessary, the input signals can he stepped down through potentiometers 30 and 3|.

For delicate comparisons, a high-stability, high-gain amplification stage can be directcoupled into each control stage.

The foregoing description is in specific terms,

and is used to control one half of the mixer proper st and many modifications will readily suggest themselves to those skilled in the art. For the true scope of the invention, therefore, reference should be had to the appended claims.

The invention described herein may be manufactured and used by or for the Government for governmental purposes, without the payment to me of any royalty thereon.

I claim:

1. In a circuit of the nature described, a first switching tube having a plate, a second switching tube having a plate, means for applying input signals one to each switching tube, a:- plate voltage supply for said tubes, similar voltage dividing means between each plate and said supply, tWo substantially similar-mixing 'tubes hav ing cathodes and control electrodes, gridbiasing means for said first mixing tube including a tap on one of the voltage dividers, and-means including a tap on the other voltage divider for I biasing the grid of said second mixing tube, a tap on said one voltage divider: at .a point more positive-than the first mentioned tap on said: voltage divider for supplying the cathode of said second mixing tube, a tap on said other voltage divider at a pointmore positive thanthe first mentioned tap thereon for supplying the cathode of said first mentioned mixing tube, and a common output for said mixing tubes.

2. An apparatus of the nature described, including a first switching tube having a plate and a control electrode, asecond switching tube having a plate and a control electrode, means for applying one signal to the control electrodeof one: of

oithe other of said mixing tubes to one of said voltage dividers thereby to maintain the lastmentioned cathode at a constantly higher poiitential than thezlastementioned control electrode,

and means for tapping the cathode of the other ofsaid mixing tubes and the control electrode of J the first-mentioned mixing tube to the other of said voltage dividers thereby to maintain the lastmentioned cathode at a constantly higher potential than the last'mentioned control electrode.

HARRY B. SMITH.

REFERENCES; CITED The'following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,755,938 Thompson Apr. 22, 1930 $185,367 Blumlein Jan. 2, 1940; 2,392,415 Soller Jan. 8, 1946 

